3GPP LTE is a communication standard employing orthogonal frequency division multiplexing (OFDM). Due to its potential for low complexity receiver implementation, OFDM is particularly attractive for high-data rate transmission.
In OFDM, the transmission bandwidth is split into equidistantly spaced orthogonal sub-bands of identical width. Orthogonality is maintained under the prerequisite that the duration of the channel impulse response does not exceed the duration of the guard interval, and if the radio propagation channel conditions vary slowly enough. Both requirements are satisfied by proper selection of system parameters, such as subcarrier spacing and guard interval duration. Then transmission of one data symbol is described by the simple equationyk,l=hk,l·xk,l+nk,l.
Here x is a transmitted symbol, h is a complex fading coefficient, n is a random noise sample, y is the corresponding received symbol, k is the OFDM subcarrier index, and l is the OFDM symbol index. The noise sample is characterized by the noise variance σn2. With different values for all different pairs of (k,l), this equation holds for all symbols in the time-frequency plane which is illustrated in FIG. 1. The above holds for a communication scheme with one transmit (Tx) antenna.
An OFDM communication scheme where multiple antennas are used both on the transmit side and the receive side is known as multiple-input multiple-output (MIMO) OFDM. In this case, each element in the time-frequency plane corresponds to the equationyk,l=Hk,l·xk,l+nk,l,where x is a vector of the transmitted symbol, H is a matrix of complex fading coefficients, n is a random noise sample vector, and y is the corresponding received symbol vector. The random noise vector is characterized by its covariance matrix Φnn.
In a multi-user system, where transmission occurs from one transmitter to multiple receivers, regions in the time-frequency plane may be assigned to different users. The 3GPP LTE standard employs this kind of orthogonal frequency division multiple access (OFDMA) in the downlink, i.e. the transmission direction from a base station to a terminal. In LTE, each element in the time-frequency plane is referred to as a resource element, and the entire time-frequency plane is divided into so-called resource blocks, which are rectangles of 12 subcarriers in the frequency direction times 6 or 7 (depending on the cyclic prefix duration mode) OFDM symbols in the time direction, as illustrated in FIG. 2.
The LTE standard describes a cellular network, where a supplied area is split into cells, each cell being equipped with a base station which serves the mobile stations in that cell; In LTE terminology a base station is referred to as an “evolved Node B” (eNB), and a mobile station or terminal is referred to as user equipment (UE). In LTE, all cells of a network operate at the same center frequency, i.e., the frequency re-use factor is 1, which means that any mobile station will experience interference from neighboring cells in the network. The interference from a neighboring cell depends on the patterns of used and non-used resource blocks in the adjacent cells. Due to processing complexity constraints and limited bandwidth resources, when a network becomes more and more loaded with users, reception at a mobile station turns more and more from a noise limited operation to an interference limited operation. In addition, the communication channel towards an interfering base station is time variant and frequency selective. Thus, when a mobile station receives signals in an LTE network, the composite of noise and interference is generally varying both in time and frequency directions.
Hence, each cell needs to be uniquely identified if a UE wishes to connect to a cell or if a UE is already connected to an LTE cell and wishes to connect to another LTE cell. For this purpose a base station transmits a cell identity (cell-ID) within the Primary Synchronization Signal (PSS) and Secondary Synchronization Signal (SSS). There are 504 unique physical layer cell identities in LTE, grouped into 168 groups of three identities. Three PSS sequences are used to indicate the cell identity within the group and 168 SSS sequences are used to indicate the identity of the group.
In a mobile radio receiver, in order to enable reliable data reception, a number of parameter estimation tasks need to be performed, e.g., time synchronization estimation, frequency synchronization estimation, channel estimation, interference level estimation, Doppler spread estimation, power delay profile estimation, feedback information estimation. PSS detection is used for slot timing detection and physical layer ID detection. SSS detection is used for radio frame detection, cyclic prefix (CP) length detection and TDD/FDD detection. SSS detection is based on coherent demodulation in the frequency domain. This is particularly a problem if a weak cell is superimposed by a strong cell having the same PSS, see FIG. 3. The channel estimate of the weak cell is severely impaired by the strong cell with the same PSS sequence. Averaging over multiple frames and low pass filtering of the channel estimate does not help in this case, since the interference is not white-noise like.
For the purpose of channel estimation reference symbols (subcarriers) are multiplexed into the time-frequency plane of the LTE downlink transmission scheme, such as illustrated in FIG. 4. Reference symbols are data symbols which are known at the receiver and are used for parameter estimation tasks.
FIG. 5 shows a state-of-the art architecture for determining the receive power of the received reference symbols. The received reference symbols are extracted either in the time domain 51 or in the frequency domain 54. In case the received reference symbols are extracted in the time domain, fine frequency correction and combining 52 is performed, and the received reference symbols are transformed to the frequency domain with a FFT 53. After frequency domain reference subcarrier (symbol) extraction 54, the received reference symbols are demodulated 56 and correlated with a sequence of reference symbols known at the receiver by computing the scalar product 45. For the purpose of noise elimination an averaging 57 can be done over a plurality of sub-frames.
All reference subcarriers are QPSK modulated for the purpose of keeping the peak to average power ratio of the transmitted waveform low. The reference signal sequence, provided in 3GPP Technical Specification 36.211 “Physical Channels and Modulation” (Release 8) can be written as:
      r          ln      s        =            1              2              ⁢          (              1        -                  2          ⁢                      c            ⁡                          (                              2                ⁢                m                            )                                      +                  j          ⁢                      1                          2                                ⁢                      (                          1              -                              2                ⁢                                  c                  ⁡                                      (                                                                  2                        ⁢                        m                                            +                      1                                        )                                                                                          where ns is the slot number with a radio frame and l is the OFDM symbol number within the slot. The pseudo random sequence c(i) is comprised of a length-31 Gold sequence, given in section 7.2 of the technical specification. The scrambling sequence generator shall be initialized with cinit=210(7*(ns+1)+/+1)(2*Ncell-ID+1)+2*Ncell-ID+NCP at the start of each frame, wherein Ncp is 1 for normal CP and O for extended CP.
Hence, the reference signal sequence also carries unambiguously one of the 504 different cell identities Ncell-id as well as the CP mode. Cell-specific reference signals (CRSs) are transmitted on all downlink subframes in a cell supporting non-MBSFN transmission. Cell-specific reference signals are transmitted on one or several of antenna ports 0 to 3.
In order to perform handover, a UE normally needs to detect the neighboring cell first and then measure their reference subcarrier receiver power (RSRP). The faster the cell search and measurement can finish, the lower is the call drop rate a user will experience. So it is crucial to reduce cell detection time and cell measurement as much as possible.